Genuine U.S. Currency has a magnetic property which is capable of being detected by a magnetic sensor. This magnetic property typically is derived from ferromagnetic particles forming one constituent of the ink used to print genuine currency.
Five major variables affecting the signal developed by the sensor utilized to detect the metal property are:
(A) Strength of the magnetic field: Each note is passed through a strong magnetic field during the sensing operation to enhance the signal developed by the sensor.
(B) Speed of the bill: The signal developed by the magnetic sensor is directly proportional to the speed of the paper currency as it passes the sensor. For this reason, detection by the sensor may be erratic when operating at slow speed.
(C) Proximity of note to sensor: If the note is not close enough to the sensor, an insufficient amount of magnetic flux will be sensed by the sensor, causing excessive false stops.
(D) Electrical characteristics: Sensitivity characteristics of the sensor are limited by its design specifications and are of no consequence to field personnel.
(E) Background noise: Motor-brush sparking, transformer windings and other sources of electromagnetic noise can act to mask out the signal produced by a genuine note and thereby reduce the effect of sensitivity of the system.
One sensor employed in the prior art for detecting genuine paper currency is described in U.S. Pat. No. 4,114,804, issued Sept. 19, 1978, and assigned to the assignee of the present invention. The sensor employed therein detects the presence of a magnetic field resulting from the interaction of the ferromagnetic ink and a magnetizing member. However, due to the above-mentioned variables, the sensitivity and hence efficiency, of the detection operation is significantly diminished.